The IEEE 802.15.4 Standard provides a radio data transceiver protocol that is optimized for low data rate, short range, low power and cost. The low power is partly a consequence of the low data rate and short range, but the standard also provides for varied low duty cycle modes in which the tranceivers will stay in a sleep state for most of the time, periodically waking up to communicate short bursts of data. In a typical 802.15.4 transceiver, two crystal oscillators are used. One crystal is a 32.768 KHz real time clock (RTC) oscillator that provides accurate timing for the system during sleep mode, so that the transceiver knows when to wake up and communicate with other devices. Because of the low frequency, this is a very low power circuit drawing only a few micro amps and allowing for a multi-year battery life. The other oscillator crystal typically comprises a 10 to 30 MHz crystal (16 MHz is the most common frequency) oscillator and is used as the reference frequency for the RF PLL frequency synthesizer and is used to operate the MCU and other digital circuitry when the device is powered on. This crystal oscillator typically consumes 400 to 800 micro amps by itself which is far too much current for the requirements of a sleep mode. The voltage-controlled oscillator (VCO) inside the RF PLL frequency synthesizer typically operates at 900 MHz to 5 GHz for 802.15.4 systems and it consumes a few mA (a few thousand micro amps), so the VCO operates only when the device is actually transmitting or receiving data.
The requirements for frequency stability vary among the three oscillators. The most stringent requirements are for the RF oscillator. The 802.15.4 Standard specifies that the oscillator must have a frequency error of less than 40 ppm over all operating conditions including temperature. This is usually achieved by phase locking to the 16 MHz crystal oscillator, so that the 40 ppm requirement transfers to that oscillator. A typical communications grade crystal could have an initial “make” tolerance of +/−10 ppm, and a temperature variation of +\−20 ppm over the industrial temperature range (−40 C to +85 C) for a total tolerance of +/−30 ppm. Some margin must be provided for other error sources such as variation in the load capacitance. In many applications, the temperature ranges would be more restrictive, allowing a looser initial tolerance. For example, the crystals mentioned above may have a temperature variation of only 10 ppm over −20° C. to +70° C.
The RTC oscillator should have good accuracy and stability. However, it doesn't need to be at 40 ppm across all operating conditions. This is particularly true for reduced function devices. One type of 802.15.4 network consists of line-powered network coordinator devices, which are always powered on, and battery-powered reduced function devices, which spend most of their time in sleep mode. In a reduced function device, there is a trade off in the tolerance of the RTC versus power consumption, since an inaccurate RTC will require the device to come out of sleep mode sooner so that it can be guaranteed to receive the beacon from the network coordinator. The network coordinator device should have better timing accuracy so that the RTC frequency errors between the slave and the coordinator are not compounded. A typical 32.768 KHz crystal has an initial tolerance of 20 ppm but a strongly parabolic frequency versus temperature curve with a maximum frequency occurring at 25 C and an error of typically −70 ppm at both −20 C and +70 C. At the extremes of the −40 C to +85 C temperature range, the frequency error will exceed 100 ppm. Therefore, in applications such as network coordinators in which sleep-mode current consumption is not important, it may be preferable to derive the RTC clock signal from the 16 MHz crystal oscillator because of its higher accuracy compared to a 32.768 KHz crystal oscillator. However, as noted earlier, this is not an option for battery-powered reduced function devices because a 16 MHz crystal oscillator typically consumes more than 100 times as much current as a 32.768 KHz crystal oscillator. These battery powered devices thus generally require both crystal oscillators, adding considerably to the cost and physical size of the transceiver. It should be pointed out that it is possible to build oscillators that do not employ a crystal as the frequency-determining element, but they do not have accuracy sufficient to meet the requirements of 802.15.4 or similar systems.
There is a need for minimizing the hardware required to implement the low duty cycle transceivers utilized in systems similar to the 802.10.4 system. Some manner of achieving this would greatly benefit the design of such systems. Furthermore, the power consumption of such transceivers may be reduced if the accuracy of the RTC clock is improved relative to that obtained from using a standard 32.768 KHz crystal oscillator as the clock source.